1. Field of the Invention
This invention is directed to optical isolation circuits, in general, and to such circuits which are fabricated using integrated circuit techniques in conjunction with silicon-on-insulator materials, in particular.
2. Prior Art
Isolation circuits are used as a means of passing a signal between two circuits without any electrically conductive path connecting the two circuits together. The electrical isolation of two circuits may be necessary and/or desirable for a number of reasons. For example, there may be different and/or varying ground potentials between the circuits. Similarly, electrical noise present in one circuit could disturb the performance of the other circuit.
One commonly used method of signal isolation is to use a light beam to couple the signal from one circuit to another. Most optical isolator-couplers use GaAs based light emitting diodes(LEDs) and silicon based photo-detectors to generate and receive the signals to be compiled.
Optical isolation circuits which use a GaAs LED and a silicon detector typically employ hybridization to make packaged opto-coupler units. That is, GaAs LEDs and silicon detectors are made in different fabrication lines because of the very different processing conditions and the different materials, such as dopants, utilized in each.
Silicon-On-Insulator (SOI) technology has emerged as a highly viable silicon technology for use in fabricating integrated circuits. SOI integrated circuits achieve component isolation via an intervening insulator rather than by back biased junctions as is the case with bulk silicon integrated circuits. Bonded wafers are one type of SOI technology which offer silicon films on SiO.sub.2 substrate which have bulk silicon quality crystallinity. Other SOI technologies which offer slightly reduced crystallinity include Zone Melt Recrystallization (ZMR), SIMOX, and Silicon-On-Sapphire. Using any of these SOI materials, it is possible to realize a totally monolithic optical isolator if silicon can be made to emit enough light to be detected.
Since the 1950's, silicon diodes have been known to emit light. However, the quantum efficiency of such a light source (2e.sup.-5 photons/carrier) is very low when compared to that of forward biased GaAs diodes. Not only are GaAs LEDs more efficient at emitting light, but the wavelength of light emitted by a GaAs diode is readily absorbed by silicon which makes silicon detectors efficient at detecting GaAs LED light. For these reasons and the fact that there was no SOI material with a high quality silicon film around at the time, the GaAs LED-silicon detector approach to optical couplers became widespread and has remained so since the 1960'so
In spite of the low quantum efficiency of silicon LEDs, it was demonstrated in 1965 that silicon could be used as an optical coupler. However, the object of that experiment was to show that light emission, and not carrier diffusion from a forward biased diode, was influencing the behavior of other diodes in the same piece of silicon. In fact, parasitic light emission from silicon devices and its effect on other devices in the same piece of silicon (i.e. increased junction leakage current) has been studied in depth.
For high speed communication between integrated circuit chips, it is desirable to use light instead of metal lines. At high signal speeds, metal lines suffer from transmission line effects and cross talk. For this reason, researchers have been trying to significantly increase the quantum efficiency of silicon light sources. For example, erbium has been used as a dopant to create optically active recombination centers in silicon. Unfortunately, these centers use effectiveness at room temperature. Other promising methods include porous silicon and carbon implanted in damaged silicon. For porous silicon, the energy of the photons emitted is above that of the silicon bandgap which means that a standard silicon PN junction diode can be used to detect the light. For the carbon implanted damaged silicon case the photon energy is below the bandgap energy of silicon.